Production of acrylic acid by the above-described acrylic acid production cycle has been widely carried out industrially. In the above-described crystallization step, acrylic acid crystal is crystallized by cooling an aqueous acrylic acid solution from the collection and/or condensation step, and this crystal is sweated and then melted to obtain a purified (product) acrylic acid. It is generally well-known that for implementation of this crystallization step, for example, dynamic crystallizer and static crystallizer are used.
For example, Patent Literature 1 (JP-B-53-41637; U.S. Pat. No. 3,621,664 corresponding to this) describes a technique where one component in a liquid mixture is sequentially concentrated by a multistep fractional crystallization method and fractionated. It has been described that in the multistep crystallization cycle, a purified molten liquid is obtained by melting the crystal obtained in a crystallization operation in a purification stage carried out in the cycle just before the cycle where crystallization step is implemented, and the purified molten liquid is heated and fed to the crystal to melt the relevant crystal.
In addition, Patent Literature 2 (JP-A-155101, U.S. Pat. No. 5,935,534 corresponding to this) describes a crystallization method where a polymerization inhibitor is added to a purified molten liquid having an equivalent purity to the crystallized crystal, and at a same time, this molten liquid is heated up to a temperature of the freezing point of the crystal or higher and circulated and fed to the crystal, to melt the relevant crystal, and the molten liquid is recovered together with the purified molten liquid added a polymerization inhibitor. As the purified molten liquid having an equivalent purity to the crystallized crystal to which polymerization inhibitor is added, use of the purified molten liquid obtained by melting the crystal obtained in a crystallization operation in a purification stage implemented in the cycle just before the cycle where crystallization step is implemented, has been described. In addition, a method where, when crystallization operation is repeated in a multistage, crystal is melted by adding a crystalline molten liquid of the same purification stage to a crystal in the same stage of another cycle, has been also described. In this literature, in order to add a polymerization inhibitor, a storage tank to store the crystalline molten liquid for the aforementioned purpose must be necessarily installed. In addition, a means to heat the crystalline molten liquid is also required.